(1) Field of the Invention
The present invention relates to a process for the preparation of porous sheets. More particularly, the present invention provides a process for the preparation of a porous sheet excellent in the moldability such as the adaptability to extrusion or drawing and the physical properties such as the water resistance, which comprises forming a composition comprising a polyolefin resin, a filler and a silicone oil into a sheet and drawing the formed sheet.
(2) Description of the Prior Art
As the conventional process for preparing a porous sheet, there is known a process in which an inorganic filler is incorporated in such a large amount as 40% by weight or more into a polyolefin resin such as polyethylene or polypropylene, the mixing is molded into a sheet and the sheet is uniaxially or biaxially drawn. The porous sheet obtained according to this process allows permeation of gases such as air and vapors such as water vapor but it hardly allows permeation of a liquid having a high polarity, such as water. The porous sheet is used as a packaging material for a dehumidifying agent by utilizing this property. Furthermore, by utilizing the property that dewing is not caused because of a high air permeability, the porous sheet is preferably used as a wall paper or a packaging material for various materials. Recently, it is expected that if a good softness is imparted to this porous sheet having such excellent properties, the porous sheet will be used in the field of simple clothing articles such as throw-away diapers and raincoats.
The porous sheet to be used in this field should have not only a high softness but also high tensile strength, high tear strength, high elongation and high water resistance, that is, the property that permeation of water is not allowed under atmospheric pressure.
As means for obtaining a porous sheet satisfying these requirements, there has been proposed a process in which linear low-density polyethylene is used as the polyolefin resin. However, a sheet obtained by incorporating a large amount of a filler into such linear low-density polyethylene, molding the mixture into a sheet and uniaxially drawing the sheet is defective in that the tear strength in the drawing direction is reduced. Furthermore, a sheet obtained by biaxially drawing the molded sheet is defective in that uniform whitening is not attained because interfacial peeling between the filler and the matrix resin is not uniformly effected, and also in that the surface strength is reduced. As means for eliminating these defects, there has been proposed a process in which the draw ratio is reduced. However, according to this process, drawing unevenness is increased and uniform whitening is not attained, though reduction of the surface strength is moderated. Trails have been made to obtain uniform whitening by incorporating a polyhydroxyl saturated hydrocarbon, an epoxy type plasticizer or a polyester type plasticizer into a polyolefin resin. However, these trails are defective in that a sufficient effect is not attained unless the additive is used in a relatively large amount and if the additive is added in a large amount, the additive bleeds out to the surface to render the surface sticky or generate a bad smell. Accordingly, sheets prepared according to these proposals are not suitable as packaging materials for food and the like or as clothing articles such as diapers, and application fields of these sheets are limited.
In the specification of U.S. Pat. No. 4,613,643, we previously proposed a process for the preparation of soft porous sheets, which comprises forming into a sheet a composition comprising (i) 40 to 80% by weight of an inorganic filler and (ii) 60 to 20% by weight of a polyolefin type thermoplastic elastomer composition comprising at least 50% by weight of a polyolefin type thermoplastic elastomer substantially free of a polar group, in which the tensile stress at a temperature of 25.degree. C. under 100% stretching (100% modulus) is 10 to 100 Kg/cm.sup.2, the elongation at break under stretching is at least 100% and the permanent strain under 100% stretching is less than 50%, and up to 50% by weight of a thermoplastic plastomer, and stretching the sheet at a temperature lower than the softening temperature of the polyolefin type thermoplastic elastomer. A porous sheet obtained according to this process has a good combination of flexiblity, air permeability and water resistance. Furthermore, in Japanese Patent Application No. 266437/84, we proposed a process comprising homogeneously mixing (i) 100 parts by weight of a mixture comprising 100 parts by weight of a crystalline polyolefin resin and 20 to 100 parts by weight of a copolymer of ethylene with other .alpha.-olefin, which has an X-ray crystalline degree of 5 to 25% and a density of 0.88 to 0.9 g/cm.sup.3, with (ii) 50 to 400 parts by weight of an inorganic filler and (iii) 0.1 to 20 parts by weight of a polyester type plasticizer and/or an epoxy type plasticizer, molding the mixture into a sheet and drawing the sheet. These processes are effective for obtaining porous sheets satisfying the above-mentioned requirements of the physical properties. However, these proceses are defective in that an expensive elastomer or copolymer should be used in a large amount, and furthermore, the latter process is defective in that a platicizer generating a bad smell has to be used. Accordingly, further improvements are desired.